Q: What does H2 Chemistry Notes: CORE IDEA 2, Topic 4 - Theories of Acids and Bases cover?
A: Compare Arrhenius, Brønsted-Lowry, and Lewis definitions while mastering pH calculations, buffer design, and titration curves for Core Idea 2 (Theories of Acids and Bases).
Acid-base theory appears across Papers 1-4, from conceptual MCQs to practical titrations. This note organises the definitions, equilibrium expressions, and exam-standard calculations required in 2026. Continue exploring linked drills at https://eclatinstitute.sg/blog/h2-chemistry-notes.
State the theory explicitly in answers. For example, “According to Brønsted-Lowry theory, ammonia acts as a base because it accepts a proton to form NHX4X+
.”
2 Quantifying Acid Strength
2.1 Ka and pKa
For a weak acid HA, the acid dissociation constant is
Ka=[HA][HX+][AX−],pKa=−log10Ka.
Smaller pKa values indicate stronger acids (greater dissociation). Always include state symbols when writing equilibrium equations, for example:
HA(aq)+HX2O(l)HX3OX+(aq)+AX−(aq).
2.2 Polyprotic Acids
Each deprotonation has its own Ka. When calculating pH, consider whether steps are independent. Typically, the first dissociation dominates, and subsequent Ka values are much smaller.
Then Ka=C−xx2. If x is much smaller than C, you can approximate C−x≈C to simplify the algebra.
After solving for x, compute pH: pH=−log10x.
4 Buffers and Henderson-Hasselbalch
Buffers resist pH change when small amounts of acid or base are added. For a weak acid buffer:
pH=pKa+log10([HA][AX−])
Use the Henderson-Hasselbalch equation only after confirming both species are present in appreciable amounts. When strong acid/base is added, adjust moles first, then recompute concentrations before applying the equation.
Design principle: Choose pKa close to the desired buffer pH (within ±1) for maximum capacity.
5 Titration Curves
Recognise four canonical curves: strong acid vs strong base, strong acid vs weak base, weak acid vs strong base, weak acid vs weak base. Key checkpoints:
Initial pH: Based on identity of the analyte.
Buffering region: Exists when a weak acid/base titrated with strong counterpart.
Half-neutralisation point:pH=pKa (or pOH=pKb).
Equivalence point: Determine species present; e.g. weak acid + strong base yields basic salt solution.
State pH values qualitatively if data not supplied (e.g. “equivalence point pH>7 due to hydrolysis of the conjugate base”).
6 Worked Example
Question:25.0mL of 0.100molL−1 ethanoic acid is titrated with 0.100mol⋅L−1 sodium hydroxide. Ka(CHX3COOH)=1.8×10−5. Calculate the pH after 12.5mL of NaOH has been added.
Solution:
Initial moles:n(CHX3COOH)=CV=(0.100molL−1)(25.0mL)=0.00250mol. Added base moles: n(NaOH)=(0.100molL−1)(12.5mL)=0.00125mol.
Statement:pH=4.74 (rounded to two decimal places).
At half-neutralisation, [AX−]=[HA], so pH=pKa-a result worth memorising.
7 Practical Considerations
Indicators: Choose based on equivalence point pH. For weak acid-strong base titrations, phenolphthalein matches the basic equivalence region.
Paper 4 tasks: When planning buffer preparation, describe stepwise calculations (moles before diluting, then the dilution formula C1V1=C2V2), and specify precise volumetric equipment (pipette, burette).
8 Common Pitfalls
Applying Henderson-Hasselbalch without confirming both conjugate pairs remain.
Forgetting to include the auto-ionisation of water for highly dilute strong acids or bases (e.g. when [HX+]<1×10X−6mol⋅L-1).
Mixing units (molarity vs mole).
Assuming equivalence point pH=7 for all titrations.
9 Drill Suggestions
Rank HClO, HClOX2, HClOX3, and HClOX4 by acid strength; justify using inductive effect and oxidation state.
Design a buffer at pH=9.2 using the NHX4X+/NHX3
Sketch titration curves for (a) HNOX3 vs KOH and (b) CHX3COOH
Strong explanations weave together the chosen acid-base model, the relevant equilibrium expression, and deliberate calculations. Keep practising with past-year guided questions and revisit the main hub at https://eclatinstitute.sg/blog/h2-chemistry-notes for further practice sets.
H2 Chemistry Notes: CORE IDEA 2, Topic 4 - Theories of Acids and Bases
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Remaining CHX3COOH=0.00125mol; formed CHX3COOX−=0.00125mol.
